Numerous processes have been proposed for the isomerization of one or more of xylenes (meta-xylene, ortho-xylene and para-xylene) to form other isomers of xylene. In many instances, the sought xylene isomer is para-xylene due to the demand for terephthalic acid for the manufacture of polyester.
In general, these xylene isomerization processes comprise contacting the xylene isomer sought to be isomerized with an isomerization catalyst under isomerization conditions. Various catalysts have been proposed for xylene isomerization. These catalysts include molecular sieves, especially molecular sieves contained in a refractory, inorganic oxide matrix. The catalysts also contain a hydrogenation metal component.
Due to the large scale of commercial facilities to produce para-xylene on an economically competitive basis, not only must a xylene isomerization process be active and stable, but it also must not unduly crack the aromatic feed so as to result in ring loss. Moreover, the isomerization processes produce by-products such as benzene, toluene, and aromatics having 9 or more carbon atoms. Often the xylene-containing feed to be isomerized also contains ethylbenzene. Ethylbenzene may be dealkylated, or the ethylbenzene can be converted by isomerization or transalkylation. Whether the isomerization process will dealkylate or will convert ethylbenzene depends upon the isomerization conditions including catalyst.
Where the ethylbenzene is sought to be dealkylated, several concerns exist. First, the dealkylation should be selective to the ethylbenzene and not cause undue loss of xylene. Second, the isomerization and ethylbenzene conversion should not result in undue production of transalkylated products such as toluene and trimethylbenzene. Third, the dealkylation should not cause the production of naphthenes that would contaminate any benzene stream separated from the product of the isomerization and ethylbenzene conversion, and thus reduce the value of the benzene.
Catalysts containing molybdenum provide advantageously low production of naphthenes. However, they do not exhibit a good balance between xylene isomerization and ethylbenzene dealkylation. A catalyst exhibiting good ethylbenzene conversion provides a low ratio of para-xylene to total xylenes. If the concentration of molybdenum is increased, some improvement can be obtained in xylene isomerization, but at a cost in ethylbenzene conversion activity.
Another difficulty with molybdenum-containing catalysts is that the xylene isomerization activity deteriorates with decreasing hydrogen partial pressure to an undesirable extent. Consequently, such a catalyst would not be useful in xylene isomerization facilities that use lower pressures.
U.S. Pat. No. 4,362,653, for instance, discloses a hydrocarbon conversion catalyst which could be used in the isomerization of isomerizable alkylaromatics that comprises silicalite (having an MFI-type structure) and a silica polymorph. The catalyst may contain optional ingredients. Molybdenum is listed as one of the many optional ingredients. U.S. Pat. No. 4,899,012 discloses catalyst for isomerization and conversion of ethylbenzene containing a Group VIII metal, lead, a pentasil zeolite and an inorganic oxide binder. U.S. Pat. No. 6,573,418 discloses a pressure swing adsorption process to separate para-xylene and ethylbenzene from C8 aromatics. Included among the catalysts disclosed for ethylbenzene isomerization are those containing ZSM-5 type of molecular sieve (Al-MFI) dispersed on silica. The catalysts contain a hydrogenation metal and listed among the hydrogenation metals are molybdenum. Suitable matrix materials are said to be alumina and silica. See example 12 which uses a molybdenum-containing catalyst for xylene isomerization.
A catalyst using molybdenum tends to generate less naphthenes than a platinum-containing catalyst. However, at comparable ethylbenzene conversions, the molybdenum-containing catalysts have been inferior to platinum-containing catalysts in isomerization activity, i.e., yields a xylene product distribution not as close to equilibrium as are the products using a platinum-containing catalyst. Hence, platinum-containing catalysts have been preferred for commercial use. Catalysts are sought that provide the combination of the low naphthene generation achievable with molybdenum catalysts with good ethylbenzene conversion and isomerization activity. Moreover, catalysts are sought that can be used in a xylene isomerization facility regardless of whether it operates at lower pressures, e.g., about 700 pKa, or higher pressures, e.g., 1500 kPa.